Dose-dependent effects of red blood cell transfusion and case mix index on venous thromboembolic events in spine surgery

Background and Objectives: Venous thromboembolic (VTE) events represent a major source of morbidity and mortality in spine surgery. Our goal was to assess whether a dose–response relationship exists between red blood cell (RBC) transfusion and postoperative VTE events among spine surgery patients. Materials and Methods: A total of 786 spine surgery patients at a single institution who received at least 1 RBC unit perioperatively were included (2016–2019). Patients were stratified based on RBC transfusion volume: 1–2 units (39.3%), 3–4 units (29.4%), 5–6 units (15.9%) and ≥7 units (15.4%). Subgroup analyses were performed after stratification by case mix index, a standardized surrogate for patients' disease severity and comorbidities. Multivariable regression was used to assess risk factors for the development of postoperative VTE events. Results: The overall VTE event rate was 2.4% (n = 19). A dose–response relationship was seen between RBC transfusion volume and VTE events (1–2 units: 0.97%, 3–4 units: 1.30%, 5–6 units: 3.20%, ≥7 units: 7.44%; p < 0.01). Similar dose–response relationships were seen between case mix index and VTE events (1.00–3.99: 0.52%, 4.00–6.99: 2.68%, ≥7.00: 9.00%; p < 0.01). On multivariable regression, larger RBC transfusion volumes (adjusted odds ratio [OR] 1.18 per RBC unit, 95% confidence interval [CI] 1.07–1.29; p < 0.01) and higher case mix index scores (adjusted OR 1.39 per unit increase, 95% CI 1.14–1.69; p < 0.01) were associated with an increased risk of thrombosis. Conclusion: Larger RBC transfusion volumes and higher case mix index scores were associated with an increased risk of VTE events. Physicians should be aware of how these dose–response relationships can influence a patient's risk of developing thrombotic complications postoperatively.